A group of proteins called molecular chaperons is a multifunctional protein, which promotes the formation of the functional structures of other proteins or maintains these structures, promotes correct association, inhibits unnecessary aggregation, protects other proteins from degradation, and promotes secretion (Non Patent Literature 1). HSP90 is a molecular chaperon as abundant as approximately 1 to 2% of all intracellular soluble proteins and is however unnecessary for the biosynthesis of the majority of polypeptides, unlike other chaperon proteins (Non Patent Literature 1). Signaling-related factors (for example, ERBB1/EGFR, ERBB2/HER2, MET, IGF1R, KDR/VEGFR, FLT3, ZAP70, KIT, CHUK/IKK, BRAF, RAF1, SRC, and AKT), cell cycle regulators (for example, CDK4, CDK6, Cyclin D, PLK1, and BIRC5), and transcriptional regulators (for example, HIF-1α, p53, androgen receptor, estrogen receptor, and progesterone receptor) are known as the main client proteins whose structure formation or stability is regulated by HSP90 through the interaction therebetween (Non Patent Literatures 2 and 3). HSP90 is deeply involved in cell proliferation or survival by maintaining the normal functions of these proteins. Furthermore, HSP90 is required for the normal functions of mutated or chimeric factors (for example, BCR-ABL and NPM-ALK) which cause carcinogenesis or exacerbation of cancer. This indicates the importance of HSP90 particularly for processes such as carcinogenesis, cancer survival, growth, exacerbation, and metastasis (Non Patent Literature 2).
The inhibition of the chaperon functions of HSP90 by specific inhibitors such as geldanamycin causes the inactivation, destabilization, and degradation of the client proteins, resulting in induction of a halt in cell proliferation or apoptosis (Non Patent Literature 4). In terms of the physiological functions of HSP90, HSP90 inhibitors are characterized in that they can simultaneously inhibit multiple signaling pathways involved in cancer survival and growth. Thus, the HSP90 inhibitors can serve as pharmaceuticals having extensive and effective anticancer activity. Moreover, from the findings that cancer cell-derived HSP90 has higher activity and higher affinity for ATP or inhibitors than those of normal cell-derived HSP90, it has been expected that the HSP90 inhibitors would serve as pharmaceuticals having high cancer selectivity (Non Patent Literature 5). Currently, the clinical development of multiple HSP90 inhibitors as anticancer agents is ongoing. The most advancing Ganetespib is under development as single agents as well as under test on the combined use with other antitumor agents such as docetaxel (Non Patent Literature 6).
Further, a new type of HSP90 inhibitor has been reported (Patent Literature 1), and HSP90 inhibitors with higher antitumor effects and reduced side effects have been desired.